1. Field Of The Invention
The present invention relates to the art of continuous papermaking by deposit of a fibrous slurry on a traveling foraminous screen. More specifically, the present invention relates to the protection of a foraminous screen carrying couch roll from galvanic deterioration.
2. Description Of The Prior Art
The historic process of manufacturing a continuous web of paper is by deposit of an aquerous slurry of wood fiber from a headbox vessel onto an endless loop of fine mesh bronze screen. The screen is generally threaded between a breast roll under the headbox and a couch or suction roll. As the screen is driven around the closed course, a portion of the water constituency of the slurry drains through the screen leaving a consolidated fibrous mat on the screen.
The couch roll about which the screen turns at the dry end of the drainage course comprises a rotating, perforated, cylindrical shell within which is mounted a stationary vacuum box. Relative to the interior periphery circle of the shell, the vacuum box interior has a sealed communication over an arcuate portion of the circle. The partial vacuum drawn within the vacuum box therefore acts through perforations in the shell and screen traveling over the sealed arc against the paper mat to draw additional water therefrom before transfer into the papermachine wet press section.
Typical couch rolls are described in U.S. Pat. Nos. 1,799,775 to E. E. Berry, 2,714,342 to E. D. Beachler and 3,171,776 to R. H. Hart et al. Representative sizes are 36 inches in diameter and 180 inches in length. Vacuum box construction is of cast iron and couch shell construction is of brass. Consequently, these rolls are generally very heavy and expensive. In addition, they are driven at high rates of speed requiring efficient anti-friction bearings between the vacuum box, the shell and the mounting pedestal.
The usual bearing arrangment for these rolls has the front extension of the vacuum box supported against rotation at the outer end on a hollow sectional pedestal which is provided with a suitable conduit between the vacuum box interior and a vacuum source. At the rear end of the suction box, it is provided with a stub shaft or journal for mounting in an anti-friction bearing assembly which, in turn, is frictionally fitted with a hollow journal depending from an adjacent head of the shell. The hollow journal of the head forms a housing for the bearing assembly and is designed to be an integral part of the head assembly so as to reduce the number of parts and machining operations.
The seal between the vacuum box and the perforated shell interior surface is by means of phenolic impregnated seal strips assisted by flooding the interior shell surface with a pumped supply of "white water" which is that liquid drawn directly from the paper mat. Such white water ranges in acidity from 4 to 6 pH and therefore constitutes an excellent electrolyte.
Notwithstanding the acidity of white water which floods all the structural surfaces within the couch interior, no difficulty has been experienced by paper makers of the past due to galvanic action between the shell and the vacuum box. Logically, this would not be anticipated since an electric short or contact through the bearings between the vacuum box and the shell would be expected. Although these bearings are efficiently lubricated, it is difficult to envision a lubrication film of such effectiveness as to maintain a galvanic action supporting EMF differential between the vacuum box and the shell.
Retrospectively, however, it may be rationalized that in past years when bronze wires were used to form the paper mat, an electrical continuity existed between the brass couch shell, the bronze wire and a stainless steel headbox to ground potential.
Consequently, when the more recent innovation of non-conductive plastic wires in lieu of bronze wires was introduced, wastage of the stainless steel headbox by galvanic action resulted. This circumstance was averted in some situations by coating the couch shell exterior with a non-conductive rubber or polymer plastic.
Now, however, it has been discovered that a rubber covered couch roll operated in combination with a non-conductive wire supports galvanic action between the shell and the vacuum box with the result that copper from the brass shell plates upon the vacuum box.
It is therefore an object of this invention to teach the construction of apparatus which prevents the wastage of a couch shell by electrolysis.
It is another object of this invention to teach the construction of apparatus which prevents the dimensional growth of the vacuum box by electroplating.